Determinants of attenuation in the envelope protein of the flavivirus Alfuy

Evolution of Zika virus in Rag1-deficient mice selects for unique envelope glycosylation motif mutants that show enhanced replication fitness

N-linked glycosylation of flavivirus envelope proteins is widely viewed as being required for optimal folding, processing and/or transit of envelope proteins, and the assembling virons, through the endoplasmic reticulum (ER) and the Golgi. Zika virus (ZIKV) has a single N-linked envelope glycan located adjacent to the fusion loop. Herein we show that independent serial passage of ZIKVNatal in Rag1 -/- mice for 223 or 386 days generated two unique envelope glycan-deficient mutants, ZIKV-V153D and ZIKV-N154D, respectively. Surprisingly, these mutants grew to titres ∼1 to 2.6 logs higher than the glycosylated parental ZIKVNatal in Vero E6 cells and human brain organoids. RNA-Seq of infected organoids suggested that this increased replication fitness was associated with upregulation of the unfolded protein response (UPR). Cell death, cellular viral RNA, and viral protein levels were not significantly affected, arguing that these glycan mutants enjoyed faster ER/Golgi folding, processing, assembly, transit, and virion egress, assisted by an upregulated UPR. Thus, ZIKV envelope N-linked glycosylation is not essential for promoting envelope folding, assembly, and transit through the ER/Golgi, since aspartic acid (D) substitutions in the glycosylation motif can achieve this with significantly greater efficiency. Instead, the evolution of glycan mutants in Rag1 -/- mice indicates that such envelope glycosylation can have a fitness cost in an environment devoid of virus-specific antibody responses. The V153D and N154D mutations, generated by natural selection in Rag1 -/- mice, have to date not been employed in orthoflavivirus envelope glycosylation studies. Instead, genetic engineering has been used to generate mutant viruses that, for instance, contain a N154A substitution. The latter may impart confounding unfavourable properties, such as envelope protein insolubility, that have a detrimental impact on virus replication. The V153D and N154D substitutions may avoid imparting unfavourable properties by preserving the surface negative charge provided by the glycan moiety in the parental ZIKVNatal envelope protein. In Ifnar1 -/- mice ZIKV-V153D and -N154D showed faster viremia onsets, but reduced viremic periods, than the parental ZIKVNatal, consistent with an established contention that such glycans have evolved to delay neutralizing antibody activity.

N-linked glycosylation of flavivirus E protein contributes to viral particle formation

In the case of the Japanese encephalitis virus (JEV), the envelope protein (E), a major component of viral particles, contains a highly conserved N-linked glycosylation site (E: N154). Glycosylation of the E protein is thought to play an important role in the ability of the virus to attach to target cells during transmission; however, its role in viral particle formation and release remains poorly understood. In this study, we investigated the role of N-glycosylation of flaviviral structural proteins in viral particle formation and secretion by introducing mutations in viral structural proteins or cellular factors involved in glycoprotein transport and processing. The number of secreted subviral particles (SVPs) was significantly reduced in N154A, a glycosylation-null mutant, but increased in D67N, a mutant containing additional glycosylation sites, indicating that the amount of E glycosylation regulates the release of SVPs. SVP secretion was reduced in cells deficient in galactose, sialic acid, and N-acetylglucosamine modifications in the Golgi apparatus; however, these reductions were not significant, suggesting that glycosylation mainly plays a role in pre-Golgi transport. Fluorescent labeling of SVPs using a split green fluorescent protein (GFP) system and time-lapse imaging by retention using selective hooks (RUSH) system revealed that the glycosylation-deficient mutant was arrested before endoplasmic reticulum (ER)- Golgi transport. However, the absence of ERGIC-53 and ERGIC-L, ER-Golgi transport cargo receptors that recognize sugar chains on cargo proteins, does not impair SVP secretion. In contrast, the solubility of the N154A mutant of E or the N15A/T17A mutant of prM in cells was markedly lower than that of the wild type, and proteasome-mediated rapid degradation of these mutants was observed, indicating the significance of glycosylation of both prM and E in proper protein folding and assembly of viral particles in the ER.

Novel approaches for the rapid development of rationally designed arbovirus vaccines

Vector-borne diseases, including those transmitted by mosquitoes, account for more than 17% of infectious diseases worldwide. This number is expected to rise with an increased spread of vector mosquitoes and viruses due to climate change and man-made alterations to ecosystems. Among the most common, medically relevant mosquito-borne infections are those caused by arthropod-borne viruses (arboviruses), especially members of the genera Flavivirus and Alphavirus. Arbovirus infections can cause severe disease in humans, livestock and wildlife. Severe consequences from infections include congenital malformations as well as arthritogenic, haemorrhagic or neuroinvasive disease. Inactivated or live-attenuated vaccines (LAVs) are available for a small number of arboviruses; however there are no licensed vaccines for the majority of these infections. Here we discuss recent developments in pan-arbovirus LAV approaches, from site-directed attenuation strategies targeting conserved determinants of virulence to universal strategies that utilize genome-wide re-coding of viral genomes. In addition to these approaches, we discuss novel strategies targeting mosquito saliva proteins that play an important role in virus transmission and pathogenesis in vertebrate hosts. For rapid pre-clinical evaluations of novel arbovirus vaccine candidates, representative in vitro and in vivo experimental systems are required to assess the desired specific immune responses. Here we discuss promising models to study attenuation of neuroinvasion, neurovirulence and virus transmission, as well as antibody induction and potential for cross-reactivity. Investigating broadly applicable vaccination strategies to target the direct interface of the vertebrate host, the mosquito vector and the viral pathogen is a prime example of a One Health strategy to tackle human and animal diseases.

The Japanese Encephalitis Antigenic Complex Viruses: From Structure to Immunity

In the last three decades, several flaviviruses of concern that belong to different antigenic groups have expanded geographically. This has resulted in the presence of often more than one virus from a single antigenic group in some areas, while in Europe, Africa and Australia, additionally, multiple viruses belonging to the Japanese encephalitis (JE) serogroup co-circulate. Morphological heterogeneity of flaviviruses dictates antibody recognition and affects virus neutralization, which influences infection control. The latter is further impacted by sequential infections involving diverse flaviviruses co-circulating within a region and their cross-reactivity. The ensuing complex molecular virus-host interplay leads to either cross-protection or disease enhancement; however, the molecular determinants and mechanisms driving these outcomes are unclear. In this review, we provide an overview of the epidemiology of four JE serocomplex viruses, parameters affecting flaviviral heterogeneity and antibody recognition, host immune responses and the current knowledge of the cross-reactivity involving JE serocomplex flaviviruses that leads to differential clinical outcomes, which may inform future preventative and therapeutic interventions.

A Small-Plaque Isolate of the Zika Virus with Envelope Domain III Mutations Affect Viral Entry and Replication in Mammalian but Not Mosquito Cells

An Asian Zika virus (ZIKV) isolated from a Thai patient that was serially passaged in Primary Dog Kidney (PDK) cells for attenuation displayed both big and small plaque-forming viruses by the 7th passage. Two small-plaque isolates were selected and purified for characterization as attenuated ZIKV candidates. In vitro growth kinetics showed significantly reduced titers for small-plaque isolates in Vero cells early post-infection compared to the parental ZIKV and a big-plaque isolate, but no significant difference was observed in C6/36 cells. Viral entry experiments elucidate that titer reduction likely occurred due to the diminished entry capabilities of a small-plaque isolate. Additionally, a small-plaque isolate displayed lowered neurovirulence in newborn mice compared to 100% lethality from infection with the parental ZIKV. Genomic analysis revealed the same three unique non-synonymous mutations for both small-plaque isolates: two on the envelope (E) protein at residues 310, alanine to glutamic acid (A310E), and 393, glutamic acid to lysine (E393K), and one on residue 355 of NS3, histidine to tyrosine (H355Y). Three-dimensional (3D) mapping suggests that the E protein mutations located on the receptor-binding and fusion domain III likely affect cell entry, tropism, and virulence. These ZIKV isolates and genotypic markers will be beneficial for vaccine development.

Novel Flavivirus Attenuation Markers Identified in the Envelope Protein of Alfuy Virus

Alfuy (ALFV) is an attenuated flavivirus related to the Murray Valley encephalitis virus (MVEV). We previously identified markers of attenuation in the envelope (E) protein of the prototype strain (ALFV₃₉₂₉), including the hinge region (E273-277) and lack of glycosylation at E154-156. To further determine the mechanisms of attenuation we assessed ALFV₃₉₂₉ binding to glycosaminoglycans (GAG), a known mechanism of flaviviruses attenuation. Indeed, ALFV₃₉₂₉ exhibited reduced binding to GAG-rich cells in the presence of heparin; however, low-passage ALFV isolates were relatively unaffected. Sequence comparisons between ALFV strains and structural modelling incriminated a positively-charged residue (K327) in ALFV₃₉₂₉ as a GAG-binding motif. Substitution of this residue to the corresponding uncharged residue in MVEV (L), using a previously described chimeric virus containing the prM & E genes of ALFV₃₉₂₉ in the backbone of MVEV (MVEV/ALFV-prME), confirmed a role for K327 in enhanced GAG binding. When the wild type residues at E327, E273-277 and E154-156 of ALFV₃₉₂₉ were replaced with the corresponding residues from virulent MVEV, it revealed each motif contributed to attenuation of ALFV₃₉₂₉, with the E327/E273-277 combination most dominant. These data demonstrate that attenuation of ALFV₃₉₂₉ is multifactorial and provide new insights for the rational design of attenuated flavivirus vaccines.

Flavivirus Envelope Protein Glycosylation: Impacts on Viral Infection and Pathogenesis

Flaviviruses encode one, two, or no N-linked glycosylation sites on their envelope proteins. Glycosylation can impact virus interactions with cell surface attachment factors and also may impact virion stability and virus replication. Envelope protein glycosylation has been identified as a virulence determinant for multiple flaviviruses, but the mechanisms by which glycosylation mediates pathogenesis remain unclear. In this Gem, we summarize current knowledge on flavivirus envelope protein glycosylation and its impact on viral infection and pathogenesis.

Envelope Protein Glycosylation Mediates Zika Virus Pathogenesis

Zika virus (ZIKV) is an emerging mosquito-borne flavivirus. Recent ZIKV outbreaks have produced serious human disease, including neurodevelopmental malformations (congenital Zika syndrome) and Guillain-Barré syndrome. These outcomes were not associated with ZIKV infection prior to 2013, raising the possibility that viral genetic changes could contribute to new clinical manifestations. All contemporary ZIKV isolates encode an N-linked glycosylation site in the envelope (E) protein (N154), but this glycosylation site is absent in many historical ZIKV isolates. Here, we investigated the role of E protein glycosylation in ZIKV pathogenesis using two contemporary Asian-lineage strains (H/PF/2013 and PRVABC59) and the historical African-lineage strain (MR766). We found that glycosylated viruses were highly pathogenic in Ifnar1^-/- mice. In contrast, nonglycosylated viruses were attenuated, producing lower viral loads in the serum and brain when inoculated subcutaneously but remaining neurovirulent when inoculated intracranially. These results suggest that E glycosylation is advantageous in the periphery but not within the brain. Accordingly, we found that glycosylation facilitated infection of cells expressing the lectins dendritic cell-specific intercellular adhesion molecule-3-grabbing nonintegrin (DC-SIGN) or DC-SIGN-related (DC-SIGNR), suggesting that inefficient infection of lectin-expressing leukocytes could contribute to the attenuation of nonglycosylated ZIKV in mice.IMPORTANCE It is unclear why the ability of Zika virus (ZIKV) to cause serious disease, including Guillain-Barré syndrome and birth defects, was not recognized until recent outbreaks. One contributing factor could be genetic differences between contemporary ZIKV strains and historical ZIKV strains. All isolates from recent outbreaks encode a viral envelope protein that is glycosylated, whereas many historical ZIKV strains lack this glycosylation. We generated nonglycosylated ZIKV mutants from contemporary and historical strains and evaluated their virulence in mice. We found that nonglycosylated viruses were attenuated and produced lower viral loads in serum and brains. Our studies suggest that envelope protein glycosylation contributes to ZIKV pathogenesis, possibly by facilitating attachment to and infection of lectin-expressing leukocytes.

Characterization of Fitzroy River Virus and Serologic Evidence of Human and Animal Infection

In northern Western Australia in 2011 and 2012, surveillance detected a novel arbovirus in mosquitoes. Genetic and phenotypic analyses confirmed that the new flavivirus, named Fitzroy River virus, is related to Sepik virus and Wesselsbron virus, in the yellow fever virus group. Most (81%) isolates came from Aedes normanensis mosquitoes, providing circumstantial evidence of the probable vector. In cell culture, Fitzroy River virus replicated in mosquito (C6/36), mammalian (Vero, PSEK, and BSR), and avian (DF-1) cells. It also infected intraperitoneally inoculated weanling mice and caused mild clinical disease in 3 intracranially inoculated mice. Specific neutralizing antibodies were detected in sentinel horses (12.6%), cattle (6.6%), and chickens (0.5%) in the Northern Territory of Australia and in a subset of humans (0.8%) from northern Western Australia.

New genotypes of Liao ning virus (LNV) in Australia exhibit an insect-specific phenotype

Liao ning virus (LNV) was first isolated in 1996 from mosquitoes in China, and has been shown to replicate in selected mammalian cell lines and to cause lethal haemorrhagic disease in experimentally infected mice. The first detection of LNV in Australia was by deep sequencing of mosquito homogenates. We subsequently isolated LNV from mosquitoes of four genera (Culex, Anopheles, Mansonia and Aedes) in New South Wales, Northern Territory, Queensland and Western Australia; the earliest of these Australian isolates were obtained from mosquitoes collected in 1988, predating the first Chinese isolates. Genetic analysis revealed that the Australian LNV isolates formed two new genotypes: one including isolates from eastern and northern Australia, and the second comprising isolates from the south-western corner of the continent. In contrast to findings reported for the Chinese LNV isolates, the Australian LNV isolates did not replicate in vertebrate cells in vitro or in vivo, or produce signs of disease in wild-type or immunodeficient mice. A panel of human and animal sera collected from regions where the virus was found in high prevalence also showed no evidence of LNV-specific antibodies. Furthermore, high rates of virus detection in progeny reared from infected adult female mosquitoes, coupled with visualization of the virus within the ovarian follicles by immunohistochemistry, suggest that LNV is transmitted transovarially. Thus, despite relatively minor genomic differences between Chinese and Australian LNV strains, the latter display a characteristic insect-specific phenotype.

Zika Virus: New Clinical Syndromes and Its Emergence in the Western Hemisphere

Zika virus (ZIKV) had remained a relatively obscure flavivirus until a recent series of outbreaks accompanied by unexpectedly severe clinical complications brought this virus into the spotlight as causing an infection of global public health concern. In this review, we discuss the history and epidemiology of ZIKV infection, recent outbreaks in Oceania and the emergence of ZIKV in the Western Hemisphere, newly ascribed complications of ZIKV infection, including Guillain-Barré syndrome and microcephaly, potential interactions between ZIKV and dengue virus, and the prospects for the development of antiviral agents and vaccines.

Recovery of West Nile Virus Envelope Protein Domain III Chimeras with Altered Antigenicity and Mouse Virulence

Flaviviruses are positive-sense, single-stranded RNA viruses responsible for millions of human infections annually. The envelope (E) protein of flaviviruses comprises three structural domains, of which domain III (EIII) represents a discrete subunit. The EIII gene sequence typically encodes epitopes recognized by virus-specific, potently neutralizing antibodies, and EIII is believed to play a major role in receptor binding. In order to assess potential interactions between EIII and the remainder of the E protein and to assess the effects of EIII sequence substitutions on the antigenicity, growth, and virulence of a representative flavivirus, chimeric viruses were generated using the West Nile virus (WNV) infectious clone, into which EIIIs from nine flaviviruses with various levels of genetic diversity from WNV were substituted. Of the constructs tested, chimeras containing EIIIs from Koutango virus (KOUV), Japanese encephalitis virus (JEV), St. Louis encephalitis virus (SLEV), and Bagaza virus (BAGV) were successfully recovered. Characterization of the chimeras in vitro and in vivo revealed differences in growth and virulence between the viruses, within vivo pathogenesis often not being correlated within vitro growth. Taken together, the data demonstrate that substitutions of EIII can allow the generation of viable chimeric viruses with significantly altered antigenicity and virulence.

IMPORTANCE

The envelope (E) glycoprotein is the major protein present on the surface of flavivirus virions and is responsible for mediating virus binding and entry into target cells. Several viable West Nile virus (WNV) variants with chimeric E proteins in which the putative receptor-binding domain (EIII) sequences of other mosquito-borne flaviviruses were substituted in place of the WNV EIII were recovered, although the substitution of several more divergent EIII sequences was not tolerated. The differences in virulence and tissue tropism observed with the chimeric viruses indicate a significant role for this sequence in determining the pathogenesis of the virus within the mammalian host. Our studies demonstrate that these chimeras are viable and suggest that such recombinant viruses may be useful for investigation of domain-specific antibody responses and the more extensive definition of the contributions of EIII to the tropism and pathogenesis of WNV or other flaviviruses.

End-point disease investigation for virus strains of intermediate virulence as illustrated by flavivirus infections

Viruses of intermediate virulence are defined as isolates causing an intermediate morbidity/mortality rate in a specific animal model system, involving specific host and inoculation parameters (e.g. dose and route). Therefore, variable disease phenotype may exist between animals that develop severe disease or die and those that are asymptomatic or survive after infection with these isolates. There may also be variability amongst animals within each of these subsets. Such potential variability may confound the use of time-point sacrifice experiments to investigate pathogenesis of this subset of virus strains, as uniformity in disease outcome is a fundamental assumption for time-course sacrifice experiments. In the current study, we examined the disease phenotype, neuropathology, neural infection and glial cell activity in moribund/dead and surviving Swiss white (CD-1) mice after intraperitoneal infection with various Australian flaviviruses, including West Nile virus (WNV) strains of intermediate virulence (WNVNSW2011 and WNVNSW2012), and highly virulent Murray Valley encephalitis virus (MVEV) isolates. We identified notable intragroup variation in the end-point disease in mice infected with either WNVNSW strain, but to a lesser extent in mice infected with MVEV strains. The variable outcomes associated with WNVNSW infection suggest that pathogenesis investigations using time-point sacrifice of WNVNSW-infected mice may not be the best approach, as the assumption of uniformity in outcomes is violated. Our study has therefore highlighted a previously unacknowledged challenge to investigating pathogenesis of virus isolates of intermediate virulence. We have also set a precedent for routine examination of the disease phenotype in moribund/dead and surviving mice during survival challenge experiments.

The Molecular Epidemiology and Evolution of Murray Valley Encephalitis Virus: Recent Emergence of Distinct Sub-lineages of the Dominant Genotype 1

Background

Recent increased activity of the mosquito-borne Murray Valley encephalitis virus (MVEV) in Australia has renewed concerns regarding its potential to spread and cause disease.

Methodology/Principal Findings

To better understand the genetic relationships between earlier and more recent circulating strains, patterns of virus movement, as well as the molecular basis of MVEV evolution, complete pre-membrane (prM) and Envelope (Env) genes were sequenced from sixty-six MVEV strains from different regions of the Australasian region, isolated over a sixty year period (1951–2011). Phylogenetic analyses indicated that, of the four recognized genotypes, only G1 and G2 are contemporary. G1 viruses were dominant over the sampling period and found across the known geographic range of MVEV. Two distinct sub-lineages of G1 were observed (1A and 1B). Although G1B strains have been isolated from across mainland Australia, Australian G1A strains have not been detected outside northwest Australia. Similarly, G2 is comprised of only Western Australian isolates from mosquitoes, suggesting G1B and G2 viruses have geographic or ecological restrictions. No evidence of recombination was found and a single amino acid substitution in the Env protein (S332G) was found to be under positive selection, while several others were found to be under directional evolution. Evolutionary analyses indicated that extant genotypes of MVEV began to diverge from a common ancestor approximately 200 years ago. G2 was the first genotype to diverge, followed by G3 and G4, and finally G1, from which subtypes G1A and G1B diverged between 1964 and 1994.

Conclusions/Significance

The results of this study provides new insights into the genetic diversity and evolution of MVEV. The demonstration of co-circulation of all contemporary genetic lineages of MVEV in northwestern Australia, supports the contention that this region is the enzootic focus for this virus.

Murray Valley encephalitis virus is the most significant cause of mosquito-borne encephalitis in humans in Australia, and can also cause neurological disease in horses. This study reports an expanded phylogenetic study of this virus and the first molecular evolutionary analysis. Of the four recognized genotypes of Murray Valley encephalitis virus, only two were found to be actively circulating (genotypes 1 and 2), and genotype 1 was dominant. Distinct genetic sub-lineages within genotype 1 were found to have recently emerged. Molecular clock analysis indicated that genotype 2 viruses are the oldest genetic lineage while genotype 1 viruses are the most recent to diverge. The co-circulation of distinct genetic lineages of this virus in northwestern Australia, comprising the oldest and youngest lineages, supports previous findings that MVEV circulates endemically in this region.

Post-translational regulation and modifications of flavivirus structural proteins

Flaviviruses are a group of single-stranded, positive-sense RNA viruses that generally circulate between arthropod vectors and susceptible vertebrate hosts, producing significant human and veterinary disease burdens. Intensive research efforts have broadened our scientific understanding of the replication cycles of these viruses and have revealed several elegant and tightly co-ordinated post-translational modifications that regulate the activity of viral proteins. The three structural proteins in particular - capsid (C), pre-membrane (prM) and envelope (E) - are subjected to strict regulatory modifications as they progress from translation through virus particle assembly and egress. The timing of proteolytic cleavage events at the C-prM junction directly influences the degree of genomic RNA packaging into nascent virions. Proteolytic maturation of prM by host furin during Golgi transit facilitates rearrangement of the E proteins at the virion surface, exposing the fusion loop and thus increasing particle infectivity. Specific interactions between the prM and E proteins are also important for particle assembly, as prM acts as a chaperone, facilitating correct conformational folding of E. It is only once prM/E heterodimers form that these proteins can be secreted efficiently. The addition of branched glycans to the prM and E proteins during virion transit also plays a key role in modulating the rate of secretion, pH sensitivity and infectivity of flavivirus particles. The insights gained from research into post-translational regulation of structural proteins are beginning to be applied in the rational design of improved flavivirus vaccine candidates and make attractive targets for the development of novel therapeutics.

The effect of riboflavin and ultraviolet light on the infectivity of arboviruses

BACKGROUND

Arboviruses are an emerging threat to transfusion safety and rates of infection are likely to increase with the increased rainfall associated with climate change. Arboviral infections are common in Australia, where Ross River virus (RRV), Barmah Forest virus (BFV), and Murray Valley encephalitis virus (MVEV), among others, have the potential to cause disease in humans. The use of pathogen reduction technology (PRT) may be an alternative approach for blood services to manage the risk of arboviral transfusion transmission. In this study, the effectiveness of the Mirasol PRT (Terumo BCT) system at inactivating RRV, BFV, and MVEV in buffy coat (BC)-derived platelets (PLTs) was investigated.

STUDY DESIGN AND METHODS

BC-derived PLT concentrates in additive solution (SSP+) were spiked with RRV, BFV, or MVEV and then treated with the Mirasol PRT system. The level of infectious virus was determined before and after treatment, and the reduction in viral infectivity was calculated.

RESULTS

Treatment with PRT (Mirasol) reduced the amount of infectious virus of all three arboviruses. The greatest level of inactivation was observed for RRV (2.33 log; 99.25%), followed by BFV (1.97 log; 98.68%) and then MVEV (1.83 log; 98.42%).

CONCLUSION

Our study demonstrates that treatment of PLT concentrates with PRT (Mirasol) reduces the infectious levels of RRV, BFV, and MVEV. The relevance of the level of reduction required to prevent disease transmission by transfusion has not been fully defined and requires further investigation. In the face of a changing climate, with its associated threat to blood safety, PRT represents a proactive approach for maintaining blood safety.

Substitution of the premembrane and envelope protein genes of Modoc virus with the homologous sequences of West Nile virus generates a chimeric virus that replicates in vertebrate but not mosquito cells

Background

Most known flaviviruses, including West Nile virus (WNV), are maintained in natural transmission cycles between hematophagous arthropods and vertebrate hosts. Other flaviviruses such as Modoc virus (MODV) and Culex flavivirus (CxFV) have host ranges restricted to vertebrates and insects, respectively. The genetic elements that modulate the differential host ranges and transmission cycles of these viruses have not been identified.

Methods

Fusion polymerase chain reaction (PCR) was used to replace the capsid (C), premembrane (prM) and envelope (E) genes and the prM-E genes of a full-length MODV infectious cDNA clone with the corresponding regions of WNV and CxFV. Fusion products were directly transfected into baby hamster kidney-derived cells that stably express T7 RNA polymerase. At 4 days post-transfection, aliquots of each supernatant were inoculated onto vertebrate (BHK-21 and Vero) and mosquito (C6/36) cells which were then assayed for evidence of viral infection by reverse transcription-PCR, Western blot and plaque assay.

Results

Chimeric virus was recovered in cells transfected with the fusion product containing the prM-E genes of WNV. The virus could infect vertebrate but not mosquito cells. The in vitro replication kinetics and yields of the chimeric virus were similar to MODV but the chimeric virus produced larger plaques. Chimeric virus was not recovered in cells transfected with any of the other fusion products.

Conclusions

Our data indicate that genetic elements outside of the prM-E gene region of MODV condition its vertebrate-specific phenotype.

Molecular characterization and phylogenetic analysis of Murray Valley encephalitis virus and West Nile virus (Kunjin subtype) from an arbovirus disease outbreak in horses in Victoria, Australia, in 2011

Virus was detected in the central nervous system (CNS) tissue of 11 horses from Victoria that died displaying neurological symptoms during an outbreak of disease in Australia in 2011. Five horses were identified as being infected with Murray Valley encephalitis virus (MVEV) and 6 as being infected with West Nile virus subtype Kunjin (WNV(KUN)). Analysis of partial sequence information from the NS5 and E genes indicated that the MVEVs within the samples were highly homogenous and all belonged to lineage I, which is enzootic to the tropical regions of northern Australia. Likewise, analysis of partial NS5 and E gene and full genome sequences indicated that the WNV(KUN) within the samples were also highly homogenous and clustered with WNV lineage 1, clade b, which is consistent with other WNV(KUN) isolates. Full genomes of 1 MVEV isolate and 2 WNV(KUN) isolates were sequenced and characterized. The genome sequences of Victorian WNV(KUN) are almost identical (3 amino acid differences) to that of the recently sequenced WNV isolate WNV(NSW2011). Metagenome sequencing directly from CNS tissue identified the presence of WNV(KUN) and MVEV within infected CNS tissue.

Characterization of virulent West Nile virus Kunjin strain, Australia, 2011

To determine the cause of an unprecedented outbreak of encephalitis among horses in New South Wales, Australia, in 2011, we performed genomic sequencing of viruses isolated from affected horses and mosquitoes. Results showed that most of the cases were caused by a variant West Nile virus (WNV) strain, WNV(NSW2011), that is most closely related to WNV Kunjin (WNV(KUN)), the indigenous WNV strain in Australia. Studies in mouse models for WNV pathogenesis showed that WNV(NSW2011) is substantially more neuroinvasive than the prototype WNV(KUN) strain. In WNV(NSW2011), this apparent increase in virulence over that of the prototype strain correlated with at least 2 known markers of WNV virulence that are not found in WNV(KUN). Additional studies are needed to determine the relationship of the WNV(NSW2011) strain to currently and previously circulating WNV(KUN) strains and to confirm the cause of the increased virulence of this emerging WNV strain.

Identification of residues in West Nile virus pre-membrane protein that influence viral particle secretion and virulence

The pre-membrane protein (prM) of West Nile virus (WNV) functions as a chaperone for correct folding of the envelope (E) protein, and prevents premature fusion during virus egress. However, little is known about its role in virulence. To investigate this, we compared the amino acid sequences of prM between a highly virulent North American strain (WNV(NY99)) and a weakly virulent Australian subtype (WNV(KUN)). Five amino acid differences occur in WNV(NY99) compared with WNV(KUN) (I22V, H43Y, L72S, S105A and A156V). When expressed in mammalian cells, recombinant WNV(NY99) prM retained native antigenic structure, and was partially exported to the cell surface. In contrast, WNV(KUN) prM (in the absence of the E protein) failed to express a conserved conformational epitope and was mostly retained at the pre-Golgi stage. Substitutions in residues 22 (Ile to Val) and 72 (Leu to Ser) restored the antigenic structure and cell surface expression of WNV(KUN) prM to the same level as that of WNV(NY99), and enhanced the secretion of WNV(KUN) prME particles when expressed in the presence of E. Introduction of the prM substitutions into a WNV(KUN) infectious clone (FLSDX) enhanced the secretion of infectious particles in Vero cells, and enhanced virulence in mice. These findings highlight the role of prM in viral particle secretion and virulence, and suggest the involvement of the L72S and I22V substitutions in modulating these activities.